The generation of electrical signals representing the coordinates of locations on electrographic devices has been the subject of investigation and study for many years. Such devices are found in computer graphics, computer aided design and computer aided manufacturing systems. For such utilization, however, the digitizers or graphics tablets representing such devices are called upon to perform with a high degree of accuracy. Additionally, in some applications the size of these devices must be limited. As an example, a digitizer may be mounted upon the surface of a cathode ray tube which displays inputs from the device.
The operation of a digitizer or graphics tablet generally involves the same manual procedures as are employed in conventional graphics design, a stylus or tracer or other suitable position locator representing a writing instrument being drawn across or selectively positioned upon the tablet surface. In turn, the electrographic device responds to the position of the stylus to generate analog paired coordinate signals which are digitized and conveyed to a host computer facility.
For the most part, graphics tablets have been fashioned as composite structures wherein a grid formed of two spaced arrays of mutually orthogonally disposed fine wires is embedded in an insulative carrier. One surface of this structure serves to yieldably receive a stylus input which is converted to coordinate signals. Various methods have been evolved for generating coordinate defining signals, as a stylus-grid interaction, for example, a magnetostrictive effect may be established between stylus and grid or a capacitive coupling effect may be evoked between these components.
Graphics tablets utilizing composite structures, while providing accurate, linear output coordinate signals necessarily are expensive to fabricate and are prone to damage in the normal course of use. Further, for many applications, it is desirable that the tablet be fabricated as a highly transparent composite sheet. Grid formations within composite structures generally preclude such a transparency feature.
Early investigators have observed the advantage of developing graphics tablets having writing surfaces formed as a continuous resistive coating. An immediately recognized advantage for this approach to tablet design resides in the inherent simplicity of merely providing a resistive surface upon a supportive insulative substrate such as glass or plastic. Further, the substrates and associated resistive coatings may be transparent to permit an expanded range of industrial applications.
The history of the development of such resistive coating type devices shows that investigators have encountered a variety of technical problems, one of which being the non-uniform nature of the coordinate readouts achieved with the coatings. Generally, precise one-to-one correspondence or linearity is required between the actual stylus or tracer position and the resultant measured coordinate signals. Because the resistive coatings cannot be developed practically without local resistance variations, for example of about plus or minus ten percent, the nonlinear aspects of the otherwise promising design approach have impeded the development of practical devices until recently. However, certain important technical approaches to utilizing the resistive surfaces have been achieved. For example, Turner discloses a border treatment or switching technique in U.S. Pat. No. 3,699,439 entitled "Electrical-Probe Position Responsive Apparatus and Method" issued Oct. 17, 1972, assigned in common herewith. This approach utilizes a direct current form of input to the resistive surface from a hand-held stylus, the tip of which is physically applied to the resistive surface. Schlosser, et al. describes another improvement wherein an a.c. input signal is utilized in conjunction with the devices and signal treatment of the resulting coordinate pair output signal is considerably improved. See U.S. Pat. No. 4,456,787 entitled "Electrographic System and Method", issued June 26, 1984, also assigned in common herewith. Position responsive performance of the resistive layer devices further has been improved by a voltage wave form crossing approach and an arrangement wherein a.c. signals are applied to the resistive layer itself to be detected by a stylus or tracer as described in U.S. Pat. No. 4,055,726 by Turner, et al. entitled "Electrical Position Resolving by Zero-Crossing Delay" issued Oct. 25, 1977, and also assigned in common herewith. Kable describes still another improvement in position responsive performance wherein an a.c. input is utilized in conjunction with a solid state switching arrangement and a computer program. A description of this improvement may be found in U.S. Ser. No. 06/665,302, U.S. Pat. No. 4,600,807, entitled "Electrographic Apparatus" also assigned in common herewith. Still another improvement is disclosed in Nakamura, et al. U.S. Ser. No. 06/664,980, abandoned, entitled "Electrographic System and Method" and also assigned in common herewith. In this approach, position responsive performance is enhanced through utilization of a computer controlled interpolative error correction procedure.
As the designs of resistive layer digitizers now reach a level of technical development permitting their practical implementation in precision computer graphics, computer aided design and computer aided manufacturing systems, further need has been exhibited for their additional refinement with respect to improvements in linearity, i.e. with respect to the accuracy of their performance. Such improvements are most necessary at the edge regions and corner regions of the active or working area of the tablet where non-linearity has been most prominent. The approach to date for accommodating edge region phenomenona which occurs adjacent electrodes placed at the edges of the resistive layer on the tablet has been to established a non-usable buffer region between the active area and the electrodes. Such a buffer region may have to be relatively wide, to obtain an acceptable level of accuracy within the active area of a graphics tablet. For applications having sufficient space the resistive layer in the tablet may be sized to provide the desired active area and the necessary buffer region. However, some applications cannot accommodate a wide buffer region. For example, in applications where a digitizer is applied to the surface of a display device such as a cathode ray tube (CRT), the digitizer may be required to fit within the boundaries of the CRT and the active area of the digitizer may have to be the same as the active area of the CRT display. In these applications the non-usable region of the digitizer can be no greater than the space between the active area and the outside edges of the CRT.